Niemand wird im Ernste behaupten wollen, dass die Entwicklung der Pflanze im freien Lande durch andere Gesetze geleitet wird, als am Gartenbeete.
No one will seriously want to maintain that plant development in the open country is guided by other laws than on the garden bed.
This sentence is of a startlingly polemical nature, if compared with the tone of the rest of Mendel’s paper, since this is of course precisely what the opponents attacked here would maintain.
laws = Gesetze See p. 3, s. 6.
on = am Literally, the preposition means “at”, which does not make much sense. It is also found in the manuscript and may simply be a misspelling for im.